Water heaters for supplying hot water to a bathroom or a kitchen are broadly classified into electric water heaters, gas water heaters, and oil water heaters. Any of these water heaters includes a portion called heat exchanger for transferring heat to water. Among the electric water heaters, a heat-pump heat-exchange type electric water heater (heat-pump water heater) in particular has recently drawn attention in terms of reduction of carbon dioxide as countermeasures for energy saving and against global warming. The heat-pump water heater operates on the principle that heat in the atmosphere is transferred to a heat medium to heat water with the transferred heat. More specifically, the heat-pump water heater is operated by a repeated operation of a cooling-heating cycle of transferring high heat, which is generated when a gas is compressed, to water through an intermediation of the heat exchanger, and then lowering the temperature of the heat medium back to an atmospheric temperature again with cool air generated when the gas is expanded. Theoretically, a larger amount of heat energy than that of input energy cannot be generated. However, the heat-pump water heater has a mechanism of using the heat in the atmosphere. Thus, a larger amount of heat energy than the amount of energy required for the operation can be used.
It is very important to constantly maintain a heat-transfer surface in a clean state because the heat exchanger transfers the heat to the water. When a wall surface is stained, an effective heat-transfer area is reduced, which in turn lowers heat-transfer performance. When the stain is further accumulated, a flow channel is obstructed in the worst case.
In a region where a large amount of hardness components (calcium ions or magnesium ions) is contained in water, in particular, there is a problem that “scale” mainly comprising carbonate crystal is deposited by heating and tends to adhere to the inner side of the heat exchanger. In a conventional water heater, opening/closing control of a discharge valve on the high-temperature water side of a plate-type heat exchanger, such as full opening of the discharge valve, full closing of the discharge valve, or 20 percent opening of the discharge value, is performed in a cycle of approximately one second. High-temperature water is thereby pulsated to vibrate plates of the plate-type heat exchanger. Residue in a low-temperature water side flow channel is thereby detached and removed (as disclosed in Patent Document 1, for example).